1. Field of the Invention
The present invention relates to an output circuit for a semiconductor device, and more specifically to an output circuit suitable for use as an output stage of an operational amplifier.
2. Description of the Prior Art
Conventional operational amplifiers will be described hereinbelow with reference to the attached drawings. FIG. 1 shows a conventional operational amplifier disclosed in Japanese Patent Laid-Open No. 1(1989)-318414. In FIG. 1, a constant current source I101 and three transistors M101, M102 and M107 form a current mirror circuit that supplies a constant current to an input and output amplifier stages. Two MOS transistors M105 and M106 construct a differential amplifier. Two input signals IN- and IN+ whose voltage levels change complementarily are input to the gates of the MOS transistors M105 and M106, respectively. The differential amplifier generates an output signal at its output terminal, or a drain of a transistor M104. The voltage at the output terminal is amplified by an output circuit provided with MOS transistors M107 to M113, and is then output to an output terminal OUT.
In the circuit as described above, an output sink current from a load (not shown) connected to the circuit flows as a drain current of the transistor M112. Here, the transistor M112 and the transistor M109 are connected to each other through a current mirror circuit of the transistors M112 and M111 and another mirror circuit of the transistors M109 and M110. The drain current of the transistor M112 is thus proportional to that of the transistor M109. The maximum drain current of the transistor M109 is the drain current of the transistor M107 operating as a current source. Further, the drain current of this transistor M107 is proportional to the output current of the constant current source I101 flowing through the current mirror circuit of the transistors M101 and M107.
Therefore, in order to increase the load drive capability by increasing the maximum output sink current at the output terminal OUT, it is necessary to increase the output current of the current source I101 and further to increase the bias current of the circuit.
FIG. 2 shows another conventional amplifier disclosed by U.S. Pat. No. 4,529,948. In this circuit, the maximum drain current of an output transistor M207 to which flows a sink current from a load (not shown) connected the circuit is decided by two output currents of constant current sources I201 and I202. Because the transistor M207 is connected to the constant current sources I201 and I202 through a current mirror circuit of transistors M207 and M203, a current mirror circuit of transistors M202 and M204, and a current mirror circuit of transistors M205 and M206.
In this amplifier circuit, in order to increase the load drive capability by increasing the maximum output sink current at the output terminal OUT, it is necessary to increase the output currents of the current sources I201 and I202 and to increase the bias current of the circuit.
FIG. 3 shows still another conventional amplifier disclosed by U.S. Pat. No. 4,284,957. In this circuit, a constant current source I301 and three transistors M301, M302, and M307 constructing a current mirror circuit provide a constant current of the circuit. Five transistors M302 to M306 construct a differential amplifier circuit. A transistor M308, a speed-up capacitor C302 and a transistor M307 construct a level shift circuit. Further, a capacitor C301 and two transistors M309 and M310 construct a phase compensating circuit. And, two transistors M311 and M312 construct an output circuit. The gate of the transistor M312 is connected to the source of the transistor M306, and the gate of the transistor M311 is connected to the drain of the transistor M308.
In this circuit, a sink current that flows from a load circuit (not shown) to the output circuit is decided by a gate voltage of the transistor M311. This gate voltage of the transistor M311 is decided by a gate voltage of the transistor M308. And, the gate voltage of the transistor M308 is an output of the differential amplifier circuit.
Suppose that the differential amplifier circuit operates under the ideal condition. When two input voltages IN+ and IN- are equal to each other (the input differential voltage is zero), an output voltage obtained at the drain node of the transistor M306 becomes equal to the drain of the transistor M305.
The gate and the drain of the transistor M305 are connected to each other. The drain voltage of the transistor M305 thus becomes a voltage obtained by subtracting a gate-source voltage Vgs from a supply voltage VDD. Therefore, the gate voltage of the transistor M311 is dependent upon the supply voltage VDD. Further, the bias currents of the two output transistors M311 and M312 are dependent upon the supply voltage VDD, with the result that the current consumption of the output transistors depends largely upon the supply voltage VDD. Further, when the threshold voltage Vth of the transistor M311 changes, its gate voltage and Vgs-Ids characteristics (the gate-source voltage v.s. the source-drain current) both change. Therefore, the bias currents of the two output transistors M311 and M312 change largely, so that their current consumption also changes largely.
As described above, in the conventional operational amplifiers as shown in FIGS. 1 and 2, in order to obtain a large sink current, since the normal current supplied from the constant current source must be increased, the current consumption increases undesirably. And, in the conventional operational amplifier as shown in FIG. 3, the bias currents of the output transistors change undesirably due to the influence of the supply voltage. Further, in the conventional operational amplifier, since no current flows to or from the output terminal, the current consumption increases in the steady state in which the potential at the output terminal is about the middle point of the supply voltage.